1. Field of the Invention
The present invention relates to a magnetic sensor wheel and in particular to a sensor wheel using a rotor position transducer for an AC or DC brushless motor.
2. Discussion of Related Art
Most early electric motors had a rotor rotatively mounted within a housing, often referred to as a stator. The rotor typically included windings which were energized or commutated by an electrical charge. The electrical charge was delivered to the coils of the rotor via a slip ring brush arrangement. The brushes were typically made from an electrically conductive carbon material which slid along an outer diameter of the rotor shaft, commonly referred to as the slip ring. The stator had permanent magnets which completed the electromagnetic field which powered the motor.
Although the slip ring arrangement of commutation is satisfactory, and is still in common use today, the slip ring brush arrangement of commutation requires that a motor periodically undergo maintenance for replacement of the brushes or repair of the slip ring. It is desirable to provide electric motors in which the brush can be eliminated. Such motors are referred to as brushless motors.
In the brushless motor, the permanent magnets of the motor are established on the rotor. The stator has poles with windings. The polarity of the pole windings is controlled by a controller which controls not only the direction of the current, but also the wave form of the current. The controller (sometimes called the driver) must be cognizant of the rotor position to properly control the current to the various windings of the stator. Two position sensing rings are required. The first position sensing ring is a coarse ring which is necessary primarily from motor start up until the motor reaches operational rotational speed.
In high performance applications, it is often desirable that the controls of the coils go beyond normal on/off control (sometimes referred to as trapezoidal control), but that the controller controls the current to the various stator coils in a sinusoidal manner. Thus, after the motor has started, there is an additional need for a fine resolution of rotor angular position. Thus, many brushless motors additionally have a fine resolution rotor shaft angular position sensors. To provide the positional data, many brushless motors are provided with a sensor wheel having two generally concentric position data indicating rings. The higher resolution position data indicator ring is typically outboard of the lower resolution or coarse position data indicator ring.
Often to achieve the resolution desired, the higher resolution ring required an outer diameter that was larger than desired, therefore requiring an enlargement of the motor casing. The enlargement of the high resolution data ring was worsened by the fact that when the sensors were Hall effect sensors there occurred magnetic cross-talk between the two data rings. To minimize the effect of cross-talk, typically a minimum spacing between the data rings was mandated.
There is an option to eliminate the magnetic cross-talk by the utilization of optical sensors. However, in most instances, optical sensing provides a cost penalty as compared with Hall effect sensors. Additionally, optical sensors are not as desirable if the motor is to be utilized in the harsh, dirty or dusty environment.
It is desirable to provide a sensor ring which provides better separation of the magnetic fields for the lower resolution data ring and the high resolution data ring by minimizing magnetic cross-talk.
It is also desirable to provide such a sensor ring wherein the costs are minimized.
To make manifest the above-noted desires a revelation of the present invention is brought forth.
In a preferred embodiment, the present invention brings forth a brushless motor having a multi poled rotor. Encircling the rotor is a coiled stator. A controller is provided to commutate the coils of the stator. The motor additionally has an angular position transducer to signal the controller of said rotor""s angular position. The transducer includes a magnetic position sensor wheel. The sensor wheel has a body defining a hub disposed about an axis of rotation of the rotor. The body also has a flange extending radially from the hub.
A first ring of magnetic material is disposed on one side of the flange between the rotor axis and a radially outer periphery of the flange. A second ring of magnetic material extends axially along a radially outer periphery of the flange. A first resolution sensor generates a first position indicative signal responsive of rotation of the first ring of magnetic material This first signal is utilized during motor start up. A second sensor generates a second high resolution position indicative signal responsive of rotation of the second ring of magnetic material. The second signal is utilized in controlling the wave form of the current which energizes the coils of the stator.
It is an advantage of the present invention to provide a sensor wheel which minimizes cross-talk between two magnetic wheels on a sensor ring.